Embodiments of the present invention relate to a method for sending data by inductive coupling, including receiving an antenna signal by way of an inductive antenna circuit in the presence of an alternating external magnetic field, extracting from the antenna signal a first periodic signal, producing a second periodic signal by way of a synchronous oscillator having a synchronization input receiving the first periodic signal, placing the oscillator in a free oscillation mode and applying to the antenna circuit bursts of the second periodic signal to generate an active load modulation magnetic field.
Embodiments of the present invention also relate to a device for sending and receiving data designed to implement this method.
Generally speaking, the present invention relates to inductive coupling communication techniques, also known as “Near Field Communications” or NFC. A communication by inductive coupling generally involves a so-called passive device and a so-called active device. The two devices are equipped with an antenna coil. The active device emits a magnetic field oscillating for example at 13.56 MHz, and sends data to the passive device by modulating the magnetic field. This magnetic field is designated below “external magnetic field”. The passive device sends data to the active device by load modulation.
The load modulation may be passive or active. Passive load modulation involves modifying the impedance of the antenna coil of the passive device at the rate of a data-carrying load modulation signal. This impedance modulation affects the impedance of the antenna coil of the active device, by inductive coupling. The active device may therefore extract from its antenna signal the load modulation signal used by the passive device, and deduce therefrom the data that the passive device sends it.
Active load modulation involves emitting bursts of alternating magnetic field, at the rate of the data-carrying modulation signal. The bursts of magnetic field are perceived by the active device as a passive load modulation. This technique was proposed by the applicant in European Patent EP 1 327 222 (U.S. Pat. No. 7,098,770B2), (see FIGS. 4A to 4E, page 8 table 4, paragraph [0074].
Compared to passive load modulation, active load modulation offers a greater communication distance and/or better data transmission in a hostile environment, for example an environment disturbed by metallic masses generating Foucault (Eddy) currents. However, active load modulation requires circuitry for driving the antenna coil and thus a current source, but consumes much less current than a continuous emission of magnetic field.
An active load modulation device cannot therefore be purely passive in terms of power supply (a purely passive device being electrically powered by the magnetic field emitted by the active device) but is nevertheless considered “passive” in that it does not emit the external magnetic field necessary for the communication.
To obtain a maximum communication distance, the active load modulation also requires that the load modulation magnetic field be in phase with the external magnetic field emitted by the active device. Phase rotations between the active load modulation magnetic field and the external magnetic field may cause undesirable fluctuations in the communication distance.
European Patent EP 1 801 741 describes an active load modulation NFC device using a phase locked loop to control the phase of the load modulation magnetic field (see e.g., FIG. 19). The phase locked loop includes a Voltage-Controlled Oscillator (VCO), a phase comparator, and a low-pass filter supplying a control voltage to the VCO. The phase comparator receives, as a reference frequency, a first periodic signal extracted from the antenna signal induced by the external magnetic field. The phase locked loop supplies a second periodic signal the phase of which is set on that of the first periodic signal. In data send mode, bursts of the second periodic signal are applied to the antenna circuit to generate the bursts of magnetic field.
When the device switches into the data send mode, the first periodic signal is no longer applied to the phase comparator and a sample hold circuit HLD maintains the control voltage applied to the VCO. The phase locked loop thus switches from a synchronous operating mode into a free oscillation mode and remains in this operating mode until the end of the data sending.
If it is desirable for the bursts of magnetic field to be in phase with the external magnetic field, the phase locked loop must have a very slight phase shift over the entire duration of the data send mode, which is at least equal to the duration of sending a data frame. In practice, the maximal phase shift tolerated over this period is generally in the order of ¼ of the period of the magnetic field oscillating at 13.56 MHz.
As an example, an ISO 14443-A frame has a duration in the order of 25.6 ms. As the frequency of the periodic signal is 13.56 MHz, the phase shift of the phase locked loop in free oscillation mode should preferably not be more than 18 ns, i.e. ¼ of the period of the magnetic field oscillating at 13.56 MHz.
Now, obtaining a stability greater than 18 ns over a duration of 25.6 ms means that the phase locked loop must offer extremely high precision, in the order of 0.7 ppm ((18×10−9/25.6×10−3)*106). Such precision requires very high quality and expensive circuitry.
It may thus be desired to provide a way of providing bursts of magnetic field having a small phase shift relative to the external magnetic field, without using extremely precise and expensive circuitry.